1. Field of the Invention
The present invention relates to a method and system for handling storage device requests within a host system with all allowed devices in use.
2. Description of the Related Art
In a hierarchical storage management system, data is stored in different types of storage devices depending upon the frequency of usage of the data. For instance, a system may include multiple storage media types to store data having different usage patterns and likelihoods of access. More frequently used data may be stored on direct access storage devices (DASD) comprised of high-performance rapid access storage devices, such as hard disk drives. Such readily accessible data is sometimes referred to as level zero volumes. Less frequently used data may be archived on slower and less expensive, demountable storage media, such as optical disks, magnetic tape cartridges, etc. Such archive volumes are referred to as level two storage.
A hierarchical storage management system provides mechanisms for migrating less frequently used data from level 0 to level 2 media. This provides more space for more frequently used data sets on the level 0 storage devices. If a host system attempts to access data sets maintained in level 2 storage, then software implemented in the host system would automatically cause the recall of the requested data from the level 2 to level 0 storage devices. The level 0 storage devices further store control data sets, an inventory, that the host systems utilize to access data from the storage devices.
Two common functions initiated by host systems in hierarchical storage management systems include migration and recall. Migration involves the movement of data from level 0 to level 2 storage to make more room for more frequently accessed data on the primary level 0 storage devices. If a host system attempts to access a data set that has been migrated to level 2 storage, then the recall request would be initiated to move the requested data sets from the level 2 storage to level 0.
International Business Machines Corporation (IBM.RTM.), the assignee of the subject patent application, provides the Data Facilities Storage Management Subsystem (DFSMS.RTM.) software which is included in the IBM MVS/ESA.TM. and OS/390.RTM. operating systems. This software allows host systems to perform hierarchical storage management operations, such as migration and recall. IBM, DFSMS, and OS/390 are registered trademarks of IBM, and MVS/ESA is a trademark of IBM. The operation and implementation of the DFSMS system are described in IBM publications "DFSMS/MVS V1R3 General Information," IBM document no. GC26-4900-04 (IBM Copyright, 1980, 1995) and "DFSMS/MVS V1R3 DFSMShsm Storage Administration Guide," IBM document no. SH21-1076-02 (IBM Copyright 1984, 1995), which publications are incorporated herein by reference in their entirety. U.S. Pat. Nos. 4,638,424 and 4,771,375, assigned to IBM and incorporated herein by reference in their entirety, describe how contentions between multiple hosts initiating migration and recall requests to the same volume are handled.
In some cases, the user specifies a maximum number of tape drives to handle recall functions. Only that many tape drives are used to handle recalls. For instance, if up to five tape drives are allowed to handle recall operations, then recall operations could be performed simultaneously with respect to five tapes. If all the tape drives allowed to handle recall requests are in use processing recall requests, then a recall request to a tape not mounted in one of the tape drives allowed to handle recall requests must wait until the host has completed processing all the queued recall requests for one of the current mounted tapes. Only after all queued recall requests have completed with respect to a mounted tape will another tape be mounted into the tape drive handling recall requests to service the pending recall requests.
The current methodology is based on a "mount optimization" approach to reduce the number of mounts by maximizing the number of recall requests processed for a mounted tape before demounting the tape to process recall requests with respect to an unmounted tape. The problem with this "mount optimization" approach is that higher priority recall requests directed toward an unmounted tape may be delayed a substantial period of time until all lower priority requests for a mounted tape are completed. With the current art, higher priority requests may be significantly delayed before a recall tape drive is made available and the lower priority requests directed toward a recall tape drive are completed.
The storage capacity of tape cartridges and other typical level 2 storage devices has increased significantly, thereby allowing an ever increasing number of data sets to be maintained on any given tape. One effect of the increase of storage capacity is that the average number of recall requests in a queue for a particular tape has increased. This in turn increases the expected delay before the tape is demounted and the tape drive made available for higher priority recall requests to unmounted tapes. Moreover, software programs that improve storage capacity utilization of tapes further increase the expected time to service a queue of recall requests for a mounted tape. For these reasons, the expected "wait time" a recall request must wait before a queue of recall requests is completed and a tape demounted has increased because the expected number of recall requests in a given queue has increased.